Sliced veneer is produced by shaving individual sheets of veneer from a log. These sheets are extremely thin (usually 0.5 to 0.7 mm in thickness). The sheet length is measured in the direction of fiber length and depends upon the slicer blade length and the length of the log. The width of the sheet of veneer, extending transversely to the fiber and the length of the sheet, depending upon the log can range from 200 to 1000 mm.
Known veneer drying processes involve guiding individual pieces of veneer between two conveyor belts, one lying above the other. According to the particular form of the belts, for example wire netting, a smoothing effect caused by the weight of the upper or covering belt may occur. This smoothing effect is, however, not sufficient in prior art belt drying processes to prevent the appearance of wavelike imperfections on the product during drying.
In roller drying in which the veneer is guided between rollers arranged one above the other, a comparatively higher pressure is applied to the veneer and because of that higher pressure a better smoothing effect can result.
Roller driers have disadvantages which are especially serious for sensitive and highly valuable thin veneers. Such veneer needs a certain inherent rigidity with which it can be fed from roller pair to roller pair. This is not available in most thin or moist veneer. When veneer shrinks during the drying process, generally by around 12%, stresses are created which particularly destroy thin veneer in this type of drying apparatus, i.e. the shrinkage while the veneer is relatively firmly held by the rollers causes tearing and crack formation.
In belt driers it is possible to shrink the laminate material between the belts to avoid crack formation. Also feed of thin, moist veneer between the belts is problem free. However, the weight of the covering belt is generally and disadvantageously not sufficient to smooth the veneer satisfactorily.
This veneer will therefore often be dried in belt driers and subsequently smoothed in a smoothing press, in order to remove wavelike imperfections developed in the product during the drying process.
In the belt drier of German patent document--Printed Application DE-AS 1 266 233, the piece to be dried is guided on a curved, looped path around staggered rollers. The web to be dried is entrained between two endless smooth wire fabric belts, which are passed zig-zag-wise in a controllable arrangement about the rollers with an elevated belt tension.
This increased tension generates a smoothing pressure which is applied to the web between the belts.
In this drying apparatus the shrinking of the veneer or plies during drying is not considered. The smoothing pressure is applied continuously to the veneer which is continuously squeezed between the belts. Therefore shrinking without cracking is not possible in this prior art apparatus. Indeed this kind of belt drier has yet to be effectively commercialized.